In an image-forming processing based on the electrophotographic system, the electrostatic latent images are formed on an inorganic photosensitive material such as selenium, selenium alloy, cadmium sulfide or amorphous silicon or on an organic photosensitive material formed by using a charge-generating agent and a charge-transporting agent. The thus formed electrostatic latent images are then developed with a toner, and the toner images formed on the photosensitive material are transferred onto a transfer sheet such as paper or plastic film, and are fixed thereon so as to obtain visible images.
The photosensitive material is positively charged or is negatively charged depending on its constitution. A toner that is electrically charged to a polarity opposite to that of the photosensitive material is used for the normal developing in which the photosensitive material that is electrically charged on the whole surface thereof is exposed to light, and the toner image is formed on the portions that have not been irradiated with light. On the other hand, a toner that is electrically charged to the same polarity as that of the photosensitive material is used for the reversal developing in which the photosensitive material is exposed to light, and the toner images are formed on the portions irradiated with light.
The toner is constituted by using a binder resin, a coloring agent, a wax and, as required, any other additives. Specifically, a charge control agent is, usually, added to impart desired charging properties (speed of charge, level of charge, stability in the charge, etc.), aging stability, and environmental stability. Addition of the charge control agent greatly improves the properties of the toner.
As the known charge control agents that can be positively charged upon friction, there can be exemplified Nigrosine dye, azine dye, copper phthalocyanine pigment, quaternary ammonium salt and a polymer having a quaternary ammonium salt on the side chain thereof. Further, as the charge control agents that can be negatively charged upon friction, there have been known a metal complex of monoazo dye, metal complexes of salicylic acid, naphthoic acid and dicarboxylic acid, copper phthalocyanine pigment, and resin containing acid components.
It is expected that there will be an increase in the demand for the colored toners. The charge control agents used for the colored toners must have a light color or no color so will not to affect the hue of the images. As the charge control agents, there can be exemplified metal complex compounds of hydroxybenzoic acid derivatives (e.g., see patent document 1), metal salt compounds of aromatic dicarboxylic acid (see patent document 2), metal complex compounds of anthranilic acid derivatives (see patent document 3), organoboron compounds (see patent document 4), biphenol compounds (see patent document 5), calix(n)arene compounds (see patent document 6), and cyclic phenol sulfides (see patent document 7), which are for use with the negatively charging toners. As the charge control agents for use with the positively charging toners, there can be exemplified quaternary ammonium salt compounds (see patent document 8).
As for the method of fixing toner images on the transfer sheet, the most widely used method may be the one based on the heat-melting. The heat-melting method can be roughly divided into the one of the contact type and the one of the non-contact type. Specifically, the heated roll fixing system of the contact type has a good heat efficiency and is capable of executing high-speed fixing, and has now been widely used in the copiers and printers for business use. However, the heated roll fixing system is accompanied by a problem in that a long time (stand-by time) is required for heating the roll up to a predetermined fixing temperature. As a means for solving this problem, an induction heating system was proposed and has now been put to practical use in some fields.
A representative example of the induction heating system is an electromagnetic induction heating system. The electromagnetic induction heating system, usually, uses an endless heating belt as the heating member in addition to using a heating roll. The heating belt uses a thin heat-resistant resin as a base layer, has a heat capacity smaller than that of the heating roll, and can be heated in a period of time shorter than that for the heating roll. In order to prevent the shading in the luster of image caused by a temperature difference, there has been proposed a method of preventing the occurrence of temperature difference by exchanging heat by bringing a metal roll into contact with the circumferential surface of the pushing roll that is installed facing the heating belt (see patent document 9).